1. Field of the Invention
This invention relates to computer memory subsystems and in particular to expandable memory subsystems which permit different size systems or offerings.
2. Description of Related Art
Prior art memory subsystems are implemented with one or more boards populated with memory array cards. In general, they provide different memory size (mega-byte) offerings. Three primary memory upgrade strategies have been used in prior-art systems. A "memory upgrade" increases the number of mega-bytes of memory in a customer's machine. The three memory upgrade strategies are different card sizes, flexible bus structure and multidrop design.
In the different card size approach, a fixed number of cards comprises each memory bank. A "memory bank" is an independently addressable set of memory cards which act in accordance during memory fetches and stores. Depopulated cards are used to construct the smaller memory offerings. A "depopulated card" is a memory card containing 1/2, 1/4, etc. of the allowable DRAM (Dynamic Random Access Memory) chips. This approach of using depopulated cards has some disadvantages of increased memory card part numbers, handling of card returns, and scrapping of depopulated memory cards or reworking of depopulated cards to increase the number of DRAMs on the card.
In a flexible bus structure a variable number of cards comprise each memory bank. The system to memory data bus width increases as more cards are plugged. The disadvantages include: reduced memory subsystem bandwidth and performance for the smaller memory offerings. Also the configurable memory data bus is difficult to implement on the system side.
In the multidrop design approach a variable number of cards comprises each memory bank. Each bit of the system to memory data bus is wired to each memory card. This approach is used in personal computers and most workstations. In high-performance systems that have high system clock rates this strategy severely limits the memory bandwidth. The increased memory data bus loading capacitance and the increased signal reflections on the data bus nets restrict the maximum rate at which data can be transferred between the system and memory cards. A "multidrop net" is a net which is wired to two or more receivers.